1. Field of the Invention
The present invention relates to a three-phase stepping motor and a method for driving such a motor for use in an OA apparatus such as a printer, a copying machine, or the like. The invention particularly relates to a three-phase stepping motor in which low vibration can be obtained, and a practical driving method for such a three-phase stepping motor in which low vibration can be obtained by an inexpensive means.
2. Description of the Related Art
Conventionally, a stepping motor using a permanent magnet for a rotor is often used for driving a rotation portion of an office machine such as a printer, a high-speed facsimile equipment, a normal-paper copying machine, or the like, because of the high efficiency of the stepping motor. A two-phase stepping motor is chiefly employed for use which requires a medium level of accuracy, while a three-phase stepping motor superior in cost performance is employed for use which requires a high level of accuracy, low vibration, low noise, etc.
As a stepping motor for use in an office machine such as a laser printer, a facsimile equipment, or the like, which requires high accuracy in positioning, a three-phase stepping motor constituted by a cylindrical permanent-magnet rotor having numbers of magnets formed cylindrically or a hybrid rotor having a permanent magnet disposed between two magnetic plates provided with numbers of pole teeth, and a stator having pole teeth formed in opposition to the surface of this rotor, is often employed because high resolution and high torque can be obtained in such a three-phase stepping motor.
Two-phase/three-phase excitation for driving a three-phase stepping motor is generally executed as shown conceptually in FIGS. 32 and 33.
FIG. 32 shows a case of two-phase excitation, and FIG. 33 shows a case of three-phase excitation.
As seen in FIGS. 32 and 33, three series circuits of switching elements, such as transistors, T.sub.11 and T.sub.12, T.sub.21 and T.sub.22, and T.sub.31 and T.sub.32 are connected between the output terminals of a DC power source E. The junction points of the respective three series circuits are connected to three terminals of a coil I wound on a main pole 31a of a first phase, a coil II wound on a main pole 31b of a second phase, and a coil III wound on a main pole 31c of a third phase, respectively. The coils I, II and III are connected into a star connection.
In the circuit of FIG. 32, when two-phase excitation is executed so that the switching elements T.sub.11 and T.sub.32 are turned on, an electric current flows as shown by the arrow in FIG. 32. As a result, the main pole 31a of the first phase is excited to be a south (S) polarity, and the main pole 31c of the third phase is excited to be a north (N) polarity.
Accordingly, the N pole of a rotor 32 is attracted to the S pole formed in the first-phase main pole 31a, and the S pole of the rotor 32 is attracted to the N pole formed in the third-phase main pole 31c, so that the rotor 32 is set to a position shown in FIG. 32.
In the circuit of FIG. 33, when three-phase excitation is executed so that the switching elements T.sub.11, T.sub.22 and T.sub.32 are turned on, an electric current flows as shown by the arrow in FIG. 33. As a result, each of the main pole 31a of the first phase and the main pole 31b of the second phase is excited into S poles, and the main pole 31c of the third phase is excited into an N pole. Accordingly, the N pole of the rotor 32 is attracted to the S poles formed in the first-phase main pole 31a and the second-phase main pole 31b, and the S pole of the rotor 32 is attracted to the N pole formed in the third-phase main pole 31c, so that the rotor 32 is set to a position shown in FIG. 33.
Similarly to the above-mentioned manner, the six switching elements are turned on sequentially, so that the state shown in FIGS. 32 and 33 is rotated to make the rotor 32 rotate clockwise sequentially.
As is understood from FIGS. 32 and 33, there is a difference in the electric current value supplied from the power source between the two-phase excitation and the three-phase excitation.
The state of torque generated in the above-mentioned excitation can be shown in a vector diagram of FIG. 34.
In FIG. 34, .tau..sub.1 designates torque generated in any one main pole having a coil mounted thereon when an electric current is applied only to that coil, and .tau..sub.2-1, .tau..sub.2-3 and .tau..sub.2-5 designate the vector sum of torque generated in the main poles sequentially when the switching elements are turned on sequentially in two-phase excitation shown in FIG. 32.
Further, .tau..sub.3-2 and .tau..sub.3-4 designate the vector sum of torque generated in the main poles when the respective switching elements are turned on sequentially in three-phase excitation shown in FIG. 33.
Accordingly, as is apparent from FIG. 34, EQU .tau..sub.2 =.sqroot.3.tau..sub.1 EQU .tau..sub.3 =2.tau..sub.1
are established.
That is, since the load circuit viewed from the power source at the time of two-phase excitation is different from that at the time of three-phase excitation, the electric current value supplied from the power source becomes different between the two-phase/three-phase excitations so that the electric current value at the time of three-phase excitation becomes 3/2 times as large as that at the time of two-phase excitation.
In addition, constant-current driving is superior to constant-voltage driving to drive a stepping motor at a high speed. Accordingly, constant-current driving is broadly used practically.
In order to obtain damping characteristics necessary for braking a rotor quickly to improve the positioning accuracy of three-phase stepping coils connected into a star-connection, a complicated damping circuit is used, or a mechanical damper or the like is used.
As for such a two-phase permanent-magnet stepping motor as mentioned above, there are those which are disclosed in U.S. Pat. No. 4,983,867, U.S. Pat. No. 5,128,570, U.S. Pat. No. 5,289,064, U.S. Pat. No. 5,386,161, and U.S. Pat. No. 5,532,531.
To realize low vibration and low noise, various proposals have been made about half-step driving or micro-step driving, as disclosed, for example, in U.S. Pat. No. 3,599,069, U.S. Pat. No. 4,663,577, and U.S. Pat. No. 5,321,340.
The above U.S. Pat. No. 3,599,069 discloses a half-step driving method based on constant-voltage driving is disclosed, the U.S. Pat. No. 4,663,577 discloses a method based on five-phase step driving, and the U.S. Pat. No. 5,321,340 discloses a half-step driving method in a star-connection of a three-phase motor.
In addition, the technique disclosed in the U.S. Pat. No. 5,321,340 is basically identical with the technique disclosed in the U.S. Pat. No. 3,599,069. According to the half-step driving method described with reference to FIGS. 1 to 5 and FIG. 8 of the U.S. Pat. No. 5,321,340, three-phase electric currents are controlled so as to be equal to each other, so that the value of an electric current supplied from a power source at the time of three-phase excitation is larger than the value of an electric current supplied from the power source at the time of two-phase excitation. Therefore, claim 2 in the U.S. Pat. No. 5,321,340 defines a technique in which the value of an electric current supplied from the power source at the time of three-phase excitation is made smaller than the value of an electric current supplied from the power source at the time of two-phase excitation to thereby make the torque substantially equal both in the three-phase excitation and two-phase excitation.
Each of the methods disclosed in the above-mentioned U.S. Patents is insufficient for optimization of magnetic paths and stator tooth width for three-terminal driving which can realize lower vibration.
In addition, the micro-step driving was expensive in its driving circuit, and the conventional half-step driving system was complicated in practical use.
Further, as described above with reference to FIGS. 32 to 34, an electric current value at the time of three-phase excitation becomes 3/2 times as large as that at the time of two-phase excitation. Accordingly, unevenness appears in driving torque. Particularly in a motor designed such that a stator has no pole teeth as shown in FIGS. 32 and 33, steps cannot be made small so that lowering in positioning accuracy or unevenness in rotation occurs more seriously because of a shortage of resolution. Accordingly, it is difficult to put such a motor into practical use.
In the case where the electric current value at the time of two-phase excitation was different from the electric current value at the time of three-phase excitation, there was a problem that electric current control became so complicated to make it difficult to perform expensive half-step driving such as two-phase/three-phase excitation.
Further, in the technique disclosed in U.S. Pat. No. 3,599,069, the driving circuit is of an old type, and complicated. On the other hand, the technique disclosed in U.S. Pat. No. 4,663,577 is mainly based on the assumption of five-phase step driving. Accordingly, if it is intended to apply this technique to three-phase full-step driving, claim 2 defining a case of star connection is not clear in its statement and fails to teach a technique for half-step driving, though claim 1 proposing a case of delta connection has a possibility to be applied to three-phase full-step driving.
In addition, the means, which is defined in claim 2 of U.S. Pat. No. 5,321,340, for making the value of an electric current supplied from a power source at the time of three-phase excitation smaller than that at the time of two-phase excitation so as to make the torque in the former case substantially equal to the torque in the latter case is practical in the constant-voltage driving. In the case of constant-current driving, however, the current detection means for making the electric current value supplied from the power source at the time of two-phase excitation different from that at the time of three-phase excitation are required to be provided at three places for the respective phases. Accordingly, there is an economical problem that a driving circuit cannot be formed inexpensively.
In order to perform positioning control in a three-phase stepping motor of star connection, a complicated damping circuit as mentioned above, a mechanical damper, or the like, is used, and there is no disclosure about an inexpensive technical means for improving the damping characteristic.
High resolution and high torque can be obtained in a three-phase motor having a cylindrical permanent-magnet rotor or a hybrid rotor, and a stator formed with pole teeth, as mentioned above. However, there is a disadvantage that a large number of harmonics are contained in field magnetic flux density generated in an air gap between the permanent magnet of the rotor and the stator, so that noise or vibration gets larger.
That is, the noise/vibration of a stepping motor is generated by a vibration torque component contained in the torque generated by the product of an excitation electric current and field magnetic flux density, so that the noise/vibration is affected by harmonic components contained in the excitation electric current and the field magnetic flux density.
While the amount of harmonic components in the field magnetic flux density is chiefly determined in accordance with the structure of the motor, the harmonic components in the excitation electric current can be reduced by micro-step driving or half-step driving such as two-phase/three-phase excitation.
However, the micro-step driving is so expensive that it is not always suitable for office machines. In addition, in a multi-pole motor as mentioned above, it is necessary to increase the number of poles to obtain high resolution. Accordingly, there was a problem that high-speed driving was difficult in constant-voltage driving.
In addition, there was another problem that the potential at a neutral point to which three-phase coils are connected fluctuates due to the excitation electric current to give influence on stable rotations of the motor.
Further, in a conventional three-phase stepping motor having, for example, six poles, if an electric current is made to flow into coils Ia and Ib for one phase as shown in FIG. 35, magnetic flux coming out from an N pole of a rotor returns to the same N pole of the rotor through an S pole of a stator and main poles of other phases as shown by a dot line in FIG. 35.
Therefore, the magnetic flux is affected by the magnetic change in the main poles due to a noise current or the like flowing in the other phases. Accordingly, there is a risk of lack of stability.
It is, therefore, required to improve the foregoing problems in the conventional techniques, and to provide a stepping motor and a driving method therefor by which half-step driving based on constant-current driving can be realized inexpensively.
It is an object of the present invention to solve the foregoing problems in the conventional techniques, and to provide a three-terminal driving type three-phase stepping motor based on a new magnetic path system by which low vibration can be obtained. In other words, it is an object of the present invention to provide a driving method of a stepping motor of three-phase star connection, including a proposal of the tooth width of the stator optimum for low vibration, and a half-step excitation system optimum for low vibration and low noise in which the three-phase stepping motor which is practical and can be formed at a low price is rotated smoothly. Accordingly, this driving method exhibits low vibration, improves damping characteristics, and has stability.